1. Field of the Invention
The teachings herein relate to thin film metrology and, in particular, to a stage for improved measurements.
2. Description of the Related Art
The use of optical metrology in semiconductor manufacturing and development has grown significantly over the past several years. The technologies provide the capability to perform measurements which yield a wide variety of critical parameters, including thickness, critical dimension, trench depth, side wall angle, doping, etc. Measurement parameters such as these were previously available only through expensive and invasive techniques such as electron microscopy cross sectional imaging. Another technique, that of optical metrology, may also be used to determine various parameters of a semiconductor.
As a review, optical metrology generally may employ a variety of techniques. Common techniques include spectroscopic ellipsometry and reflectometry. Other types of optical metrology systems include those employing Raman spectroscopy, dark field and bright field wafer inspection, etc. In spectroscopic ellipsometry, and with reference to FIG. 1, an incident beam of measuring light 5 illuminates a target wafer 10 at an angle, θ. The angle, θ, is of a value that is other than normal to a surface of the wafer 10. Interrogation of the wafer 10 with measuring light 5 results in a spot (not shown). The spot resembles an ellipse. In reflectometry, and with reference to FIG. 2, the incident beam of measuring light 5 illuminates the target wafer 10 normal to the surface of the wafer 10. In most optical systems, reflectometry results in a circular spot.
The optical systems used in spectroscopic ellipsometry and reflectometry to illuminate the sample and collect the optical spectra generally make use of and control various wavelengths and other parameters to improve measurement results. Regardless of which type of optical system is used, reflected measuring light must be analyzed to determine properties of the target wafer 10. Analysis typically includes use of an optical model specifically created for the target being measured.
Advantageously, optical metrology provides information real-time using an in-linetool. A further benefit of optical metrology is that the technique does not involve contact with the sample (e.g., a semiconductor or a wafer). Unfortunately, several types of long-term measurements relying upon optical metrology can expose samples to environmental effects (such as particle contamination) and result in poor measurement results. Quite often, the wafers are placed face up on the stage with no or little protection against contaminants. Desorber techniques can be used to remove some of the contaminants and temporarily reverse the contamination impacts. However, use of desorber techniques can endanger integrity of some materials (such as photo resist) and may have irreversible consequences for product wafers. Hence, tighter manufacturing controls require new methods to promote fast and non-invasive measurements under well controlled conditions.
What are needed are techniques for improved measurement accuracy and precision during optical metrology. Preferably, the techniques provide for reducing or blocking wafer contamination during the various measurement processes.